In recent years, chip type electronic parts used for surface mounting, such as ICs, transistors, diodes, condensers, piezoelectric resistors and the like have been supplied by being contained in packaging materials consisting of a plastic carrier tape having at given intervals pockets formed by embossing so as to accomodate chip type electronic parts of particular shape and a cover tape heat-sealable to the carrier tape. The electronic parts contained in the packaging materials are automatically taken out after peeling the cover tape of the package, and are mounted on the surface of an electronic circuit substrate.
The strength when the cover tape is peeled off from the carrier tape is called "peel-off strength". When the peel-off strength was too low, there was a problem that the cover tape was separated from the package at the time of the package transfer and the contents, i.e., the electronic parts fell off. When the peel-off strength was too high, there occurred a phenomenon that the carrier tape underwent vibration in peeling the cover tape and, as a result, the electronic parts jumped out of the pockets right before their mounting, i.e., jumping trouble.
As a material for the carrier tape, there have conventionally been used polyvinyl chloride (PVC) sheet, polystyrol sheet, polyester (PET) sheet, polycarbonate sheet and acrylic sheet, all of easy sheeting. Meanwhile, as a cover tape heat-sealable to the carrier tape, there have generally been used a composite film obtained by laminating a heat-sealable polyethylene-modified film or a heat-sealable ethylene-vinyl acetate copolymer (EVA)-modified film to a biaxially oriented polyester film, a PVC sheet or a styrol type sheet. In the conventional cover tape, however, the peel-off strength was greatly affected by the sealing conditions such as sealing temperature, sealing pressure and the like, and the fluctuation in sealing conditions made it difficult to control the peel-off strength in an appropriate range as mentioned above; further, the peel-off strength was also affected by the temperature and humidity during the storage of the conventional cover tape or during the storage after sealing, whereby the peel-off strength was increased or decreased with the lapse of time and, in some cases, deviated from an appropriate range.
With the significant improvement in surface mounting technique in recent years, electronic parts to be surface-mounted have come to be made in chips of higher capability and smaller size. In such a movement, the electronic parts, when transferred in a package, have undergone vibration and contact with the embossed inner surface of the carrier tape of the package or the inner surface of the cover tape of the package, and the resulting friction generated static electricity; static electricity has been also generated when the cover tape was peeled off from the carrier tape; this static electricity has generated sparks to cause destruction and deterioration of the electronic parts. Thus, the electronic parts have had troubles due to static electricity, and it has been a most important task to develop an antistatic measure for packaging materials such as carrier tape and cover tape.
The antistatic treatment for a carrier tape has hitherto been effected by incorporation or coating of carbon black into or on the cover tape material used, and the effect has been satisfactory. However, no sufficient measure has yet been taken for the antistatic treatment for a cover tape, and currently there is merely effected, for example, coating of an antistatic agent or a conductive material on the outer layer of the cover tape. This treatment, however, has not been sufficient for the protection of electronic parts to be contained by sealing, because the treatment is applied to the outer side of the cover tape, and particularly the treatment was ineffective for the static electricity generated by the contact of the inner surface of the cover tape with the electronic parts. The antistatic treatment for the inner surface of the cover tape, i.e. the adhesion layer of the cover tape can be effected by coating or incorporation of an antistatic agent on or into the adhesion layer. In this approach, however, the antistatic agent incorporated into the adhesion layer has bled onto the inner surface of the cover tape and invited unstable sealing and many troubles due to poor sealing; moreover, the antistatic effect has decreased with the lapse of time, has been greatly affected by the conditions under which the package was used, i.e., temperature and humidity, particularly humidity, and has significantly decreased under a low humidity such as 10% R.H. (relative humidity); thus, no sufficient effect has been obtained. Meanwhile, the incorporation of a conductive material into the adhesion layer has been technically difficult, because the adhesion layer has been formed by laminating a film or the like to an outer layer; moreover, the incorporation has significantly reduced the transparency of the resulting cover tape, making the cover tape usability questionable. The coating of a conductive material on adhesion layer has not been effected, because the selection of a binder stably bondable to the carrier tape has been difficult, and because the adhesion layer is covered and hidden by the coating.